Declining levels and duration of passively acquired maternal antibodies prompted a Danish trial to test the feasibility of advancing administration of the first measles, mumps, and rubella vaccine (MMR1) from 15 to 6 months of age. A trial-embedded qualitative study aimed to understand parents’ (N = 24) and health professionals’ (N = 11) attitudes about the measles, mumps, and rubella vaccine (MMR) in general and about advancing MMR1 administration. Overly positive parent attitudes were contrasted by members of a vaccine-skeptical organization including parents considering that their child was seriously vaccine-injured long ago. Parents’ attitudes to advancing MMR1 mirrored their attitudes about the MMR vaccine in general, with four positions along a continuum of trust in the healthcare system: unquestioning trust, acceptance after careful consideration, challenging indecisiveness, and defensive rejection. Low tolerance was identified between vaccine supporters and vaccine opponents. Parents of children with perceived serious vaccine-related injuries described lifelong unresolved feelings of guilt. Supporters of advanced MMR1 saw it as a timely and convenient administration of a well-known vaccine, whereas opponents feared it would disturb the children’s immature immune systems and emphasized difficulties in recognizing side effects so early in life. Health professionals were supportive of advancing the MMR1 vaccine and they carefully challenged the parents. Current MMR vaccine supporters show readiness to advance MMR1 administration.